1. Field of the Invention
The present invention relates to a controller for a motor-driven power steering mechanism which assists steering operation of a steering wheel by means of the drive force of an electric motor.
2. Description of the Related Art
FIGS. 6 to 8 show a typical motor-driven power steering mechanism and a conventional controller for the motor-driven power steering mechanism.
FIG. 6 is an explanatory view schematically showing the overall structure of the motor-driven power steering mechanism; FIG. 7 is a an explanatory view showing a torque-sensing mechanism provided in the motor-driven power steering mechanism shown in FIG. 6; and FIG. 8 is a block diagram of the conventional controller for the motor-driven power steering mechanism.
First, the overall structure of the motor-driven power steering mechanism will be described with reference to FIG. 6.
A steering wheel 70 is fixed to a shaft 61, which is connected to a torque-sensing mechanism 60. To the torque-sensing mechanism 60 is connected a shaft 75 having a reduction gear 71a provided thereon. The reduction gear 71a is in meshing engagement with a reduction gear 71b fixed to an output shaft of an electric motor 50. The shaft 75 is also connected to a steering gear 72. Wheels 73 to be steered are attached to the opposite ends of a rod 74, which is connected to the steering gear 72.
Next, the structure of the torque-sensing mechanism 60 will be described with reference to FIG. 7.
The torque-sensing mechanism 60 comprises the shaft 61, which is hollow and whose lower portion penetrates an upper portion 62a of a housing 62. The shaft 75 penetrates a lower portion 62b of the housing 62, and the above-described reduction gear 71a (see FIG. 6) arranged in meshing engagement with the reduction gear 71b is attached to the shaft 75.
A torsion bar 65 is accommodated within the interior of the shaft 61. The upper end of the torsion bar 65 is coupled with the shaft 61 by use of a pin 66, and the lower end of the torsion bar 65 is in spline-engagement with an inner portion of the shaft 75.
That is, the torque-sensing mechanism 60 is configured such that when a steering torque is transmitted to the shaft 61 upon operation of the steering wheel 70, the torsion bar 65 is twisted, resulting in generation of a relative displacement between the shaft 61 and the shaft 75.
Two paired sensor rings 67 formed of a magnetic material are disposed within the housing 62 to surround the shaft 61 One of the sensor rings 67 is secured to the shaft 61, and the other sensor ring 67 is secured to the shaft 75. A sensor coil 68 is provided within the housing 62 at such a position that the inner circumferential surface of the sensor coil 68 faces the outer circumferential surfaces of the sensor rings 67
When a relative displacement is produced between the shafts 61 and 75, the amount of overlap between the end surfaces of the sensor rings 67 changes, with the result that the inductance of the sensor coil 68 changes. Thus; a signal representing steering torque (hereinafter referred to as a xe2x80x9ctorque sensor signalxe2x80x9d) is obtained.
The sensor coil 68 is electrically connected to a controller 100 for the motor-driven power steering mechanism (see FIGS. 6 and 8).
Next, the electrical configuration of the controller 100 will be described with reference to FIG. 8.
The controller 100 includes an interface circuit (hereinafter referred to as an xe2x80x9cI/F circuitxe2x80x9d) 69, which receives the torque sensor signal and converts it to a torque signal representing the steering torque. Two microcomputers; i.e., a microcomputer 80 and a microcomputer 90, are connected to the I/F circuit 69. The microcomputer 80 includes a torque calculation section 81 and a motor control section 82. The torque calculation section 81 receives the torque signal from the I/F circuit 69 and calculates the steering torque. The motor control section 82 outputs to a drive circuit 83 a control signal corresponding to the steering torque calculated by the torque calculation section 81. The drive circuit 83 supplies drive current to the electric motor 50 in accordance with the control signal output from the motor control section 82.
The microcomputer 90 includes a torque calculation section 91, which calculates the steering torque in a manner similar to that employed in the torque calculation section 81 of the microcomputer 80. The microcomputer 90 further includes a torque monitor section 93, which compares the steering torque calculated by the torque calculation section 81 of the microcomputer 80 with the steering torque calculated by the torque calculation section 91, in order to detect a difference there between. When the difference is determined to have exceeded a predetermined level one time, the torque monitor section 93 determines that the controller 100 has come into an anomalous-state. The microcomputer 90 further includes a current monitor section 92, which detects an anomalous-state of the electric motor 50 by monitoring motor current flowing through the electric motor 50.
The operation of the controller 100 will now be described.
When a steering torque is applied to the steering wheel 70 (FIG. 6), the torsion bar 65FIGS. 7) twists, resulting in generation of a relative displacement between the shaft 61 and the shaft 75. As a result, the overlap between the end surfaces of the sensor rings 67 changes, and thus, the inductance of the sensor coil 68 changes. This change in inductance is detected, as a torque sensor signal, by the I/F circuit 69 of the controller 100 (FIG. 8) and is converted to a torque signal corresponding to the steering torque. Subsequently, the torque signal is sent to the torque calculation section 81 of the microcomputer 80, in which the steering toque is calculated on the basis of the torque signal.
Subsequently, a torque command value corresponding to the calculated steering torque is output to the motor control section 82, which in turn outputs to the drive circuit 83 a control signal corresponding to the torque command value. The drive circuit 83 supplies drive current to the electric motor 50 in accordance with the control signal, so that the electric motor 50 rotates.
Rotation of the electric motor 50 is transmitted to the shaft 75 via the reduction gears 71a and 71b in order to rotate the shaft 75. Thus, rotational torque of the shaft 75, i.e., steering toque, is increased in order to assist the steering operation.
When the torque monitor section 93 or the current monitor section 92 detects an anomalous-state, an anomaly signal is sent to the motor control section 82, in response to which the motor control section 82 stops the issuance of the control signal in order to stop control of the electric motor 50.
However, the controller 100 is configured to determine occurrence of an anomalous-state through only one-time detection of a state in which the difference obtained through the comparison operation of the torque monitor section 93 exceeds a predetermined level, and to stop the control of the electric motor 50 instantaneously after the detection of the anomalous-state.
Therefore, there is a possibility of the control of the electric motor 50 being stopped even when the steering torque value changes due to external noise which enters the electrical system extending from the torque sensing mechanism 60 to the controller 100 or external noise which enters the controller 100 itself.
That is, the conventional controller has a drawback of insufficient reliability in terms of anomalous-state judgment, or a difficulty in properly judging whether the control of the electric motor 50 is truly stopped due to an anomalous-state of the controller 100.
In view of the foregoing, an object of the present invention is to provide a controller for a motor-driven power steering mechanism which has improved reliability in terms of anomalous-state judgment.
To achieve the above object, the present invention provides a controller for a motor-driven power steering mechanism including a steering mechanism, a steering wheel connected to the steering mechanism, a torque-sensing mechanism for sensing steering torque applied to the steering wheel, and an electric motor for producing an assisting steering torque. The controller comprises a control section for controlling the electric motor on the basis of a control signal corresponding to the steering torque sensed by the torque-sensing mechanism; and an anomalous-state judgment section which receives the control signal output from the control section, determines a degree of continuity of a state in which the control signal assumes an anomalous level, and judges that the control section is anomalous when the determined degree of continuity reaches a predetermined level.
In the controller of the present invention, the degree of continuity of a state in which the control signal assumes an anomalous level is determined, and the control section is judged to be anomalous when the determined degree of continuity reaches a predetermined level, unlike the conventional controller in which the control section is judged to be anomalous immediately upon one-time detection of the control signal having reached a preset value. Therefore, reliability of anomalous-state judgment can be improved.
The anomalous-state judgment section is preferably configured to judge that the control section is anomalous when the number of times the received control signal deviates from an allowable range has reached a predetermined number. Alternatively, the anomalous-state judgment section may be configured to judge that the control section is anomalous when a period of time over which the received control signal falls out of the allowable range has reached a predetermined length.
The control section is advantageously formed of a single microcomputer, and the degree of continuity is determined by means of a digital signal processor. In this case, since the degree of continuity is determined by use of the digital signal processor, production cost of the controller can be reduced as compared with a controller in which the degree of continuity is determined by use of a microcomputer.
Preferably, the degree of continuity is determined on the basis of an average level of the control signal received during a predetermined period of time.
In a configuration such that the difference between a certain maximum value and a subsequent maximum value of the control signal is calculated, and an anomalous-state is judged to occur when the calculated difference exceeds a predetermine value, the difference increases through one-time generation of a large maximum value, resulting in erroneous detection of an anomalous-state. By contrast, in the controller according to the present invention, since the degree of continuity is determined on the basis of an average level of the control signal received during a predetermined period of time, such an erroneous detection can be avoided.
Advantageously, the anomalous-state judgment section comprises an informing device for informing an anomalous-state of the control section detected by the anomalous-state judgment section. Advantageously, the anomalous-state judgment section comprises a control prohibition section for stopping control of the electric motor when the anomalous-state judgment section detects an anomalous-state of the control section.